DTI Processing - Voxel-based versus tract-based diffusion imaging

Sunday, October 21, 2012
Do Tromp
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The development of diffusion magnetic resonance imaging (dMRI) enabled the research of white matter micro- and macro-structure in vivo. DMRI measures the magnitude and orientation of water diffusion. This is done in multiple directions to calculate the three dimensional representation of the water diffusion profile. Gray matter has predominantly isotropic (soccer ball shaped) water diffusion, while dense white matter tracks have highly anisotropic (rugby ball shaped) diffusion of water pointing in the direction of the fiber bundle.
The measure most commonly used to characterize directional diffusion is fractional anisotropy (FA). This measure gives a value between 0 and 1 to indicate the fraction of diffusion that is in the longitudinal direction compared to the proportion of diffusion in both transverse directions. Other measures that can be used are axial diffusivity (AD), radial diffusivity (RD) and mean diffusivity (MD).

Voxel-Based Morphometry

There are two main methods of analyzing diffusion images. The first is voxel‐based analyses (VBA) , which is specifically suited for whole brain analysis. It is a voxel wise method to statistically compare local anisotropy values for the whole brain between different subjects. It has to be kept in mind that this method should correct for multiple comparisons. One way to reduce the number or comparisons is to use an atlas based segmentation methods to selectively investigate white matter areas of interest.

Tract-Based Analysis

The second method is called tract‐based analysis. It uses the more anisotropic tensors to form streamlines of tensors leading to estimations of white matter fiber tracts. A region of interest is used as seed region from where the fibers are traced. For each tract mean FA values can be calculated. These values per tract can be compared across groups to investigate structural connectivity.

VBA and fiber tractography are two methods using a fairly different approach in dMRI. In VBA the whole brain is investigated, but the method relies heavily on effective registration between subjects. When regions of abnormal FA values do not map onto each other correctly this will greatly reduce the likelihood to find significant results. In tract‐based analyses tracts can be delineated without relying on subject registration. Although specific a priori regions of interest or specific tracts need to be selected for comparison.

About This Website

Diffusion-imaging.comgives a comprehensive overview of available software, analyzing methods and research possibilities. Providing background on the tools, methods and software to effectively analyze DTI data. Posts include step-by-step processing and tractography tutorials.

About the developer

Do grew up and went to college in the Netherlands, where she also received her masters degree in Neuroscience & Cognition. She is currently a neuroscience graduate student in the US. Her interest is focused on understanding the neural substrates that underlie normal and abnormal brain functioning. She uses state of the art imaging methods, like magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) to examine alterations in white-matter structure.

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